Meteor from Vesta's surface shows a protoplanet with magnetic history.

Models of our solar system's formation suggest that the inner rocky planets were built in stages, as small objects combined to create planetesimals, which merged to protoplanets. Collisions among these built the planets we're familiar with. Although this process happened over four billion years ago, it may still be possible to understand this era. Some evidence implies that Saturn's moon Phoebe may be a planetesimal, while some of the largest asteroids appear to be protoplanets. By studying them, we can look back to the earliest stages of our Solar System's history.

Thanks to NASA's Dawn mission, we recently got a better look at one of these protoplanets, called Vesta. The spacecraft's visit revealed a surprising amount of volatile material and features that suggest the planet's interior was once sufficiently hot for it to become differentiated, with different materials layered based on their density. Now, a piece of Vesta that has fallen to Earth suggests that not only was the interior once molten, but it produced a spinning dynamo that generated a magnetic field.

One of Vesta's most dramatic features is a large crater on its south pole, where an impact gouged a huge chunk out of the asteroid. This impact has ensured that Vesta populated the inner solar system with plenty of rocks, some of which have struck the Earth as meteorites. Based on their chemical similarities, the HED (howardite–eucrite–diogenite) meteorites appear to have originated on Vesta.

In 1981, an HED meteor was found at Allan Hills in Antarctica, and analysis shows that it probably originated on Vesta. Although its exterior had been partly melted during its fall through our atmosphere, its interior largely preserved the crystalline structure that had originally formed on Vesta. And that structure also preserved hints that it had formed in a magnetic field.

In fact, there were indications of three magnetic fields, but two of these were uniform across the sample, which suggests they were induced after the meteor had formed. The third was aligned with various sub-structures within the meteorite, and appears to have been generated back when the rock it was from was partly melted (probably by another impact). Dating using Argon isotopes shows that this probably took place around 3.7 billion years ago.

That's too late for the rock to have formed under the influence of any of the magnetic fields that were prevalent early in the Solar System's history. But it's also long after the interior of Vesta should have solidified, given that it's too small to have generated or retained enough heat for that long. In any case, an active dynamo would have generated a much stronger magnetic field in the sample.

What the authors propose is that a weak magnetic field, somewhere between 0.01 and four microTesla, has been locked in place in the crust of Vesta. This implies that, while active, the asteroid's inner dynamo generated a field with a strength between 10 and 100 microTesla. That is roughly comparable to the one currently measured on Earth.

If the crust does preserve the signature of Vesta's long-silent magnetic dynamo, then it should still be present. And the authors say that there is some evidence that it is. The surface of Vesta appears to show less indication of space weathering than other bodies of a similar age, which suggests something is deflecting the solar wind. At the distance Vesta orbits, the authors calculate that it would only take a field of 0.2 microTesla to deflect the solar wind—well within the range they calculate from the meteor.

The findings add to the evidence that, in many ways other than size, the protoplanets of our solar system looked a lot like regular planets do. It also shows we might need to think a bit more about what sets the lower bound on the size of a dwarf planet.